And the winner is...

... shite, I got it wrong.
It's actually at the time of the winter solstice that the golden rays of the sun enter the chamber at Newgrange.

Bollix, that's one round trip to county Louth (dangerously close to Drogheda) for nothing. I was six month late (or early, depending on whether you are a calendar half full or half empty sort of person).

And anyway, I have discovered that there is a lottery to gain access to Newgrange between 19 December and 23 December.
Many are those who seek the privilege to see the curtain of dark winter mist get marginally brighter, from within a damp cold Neolithic pile.

An online search has even retrieved a couple of photoshopped images by tricksters, of golden sun rays entering the chamber. Yeah, roigh...